1. Field of the Invention
This invention pertains to spray-acid cleaning during semiconductor fabrication and, more particularly, a durable, multi-piece rotor for spray acid tools.
2. Description of the Related Art
Semiconductor devices, or microchips, are manufactured from wafers of a substrate material. Layers of materials are added, removed, and/or treated during fabrication to create the integrated, electrical circuits that make up the device. The fabrication essentially comprises four basic operations. The four operations are:
layering, or adding thin layers of various materials to a wafer from which a semiconductor is produced;
patterning, or removing selected portions of added layers;
doping, or placing specific amounts of dopants in the wafer surface through openings in the added layers; and
heat treatment, or heating and cooling the materials to produce desired effects in the processed wafer.
Although there are only four basic operations, they can be combined in hundreds of different ways, depending upon the particular fabrication process. See, e.g., Peter Van Zant, Microchip Fabrication A Practical Guide to Semiconductor Processing (3d Ed. 1997 McGraw-Hill Companies, Inc.) (ISBN 0-07-067250-4).
Wafers under fabrication frequently require a Chemical Mechanical Polishing (xe2x80x9cCMPxe2x80x9d) process to reduce topographical variance and achieve a planarity sufficient to meet the challenging lithography requirements of microprocessor manufacturing. CMP, by microelectronic fabrication standards, is an inherently dirty process. This production step lowers wafers, device side down, onto a rotating table saturated with slurry. Water and slurry between the pad on the table and the wafer surface provide the chemical catalyst in CMP. Downward pressure applied to the back of the wafer and the aggregate suspended in the slurry solution between the wafer and the rotating table provide the mechanical catalyst in CMP.
Successfully removing slurry residuals following CMP processing aids in the successful manufacturing of operational integrated circuits. This post-polish, wafer surface conditioning consists of a chemical clean and/or a mechanical clean to reduce contamination to at least a minimally acceptable level, a direct correlation to a good die yield. Chemical cleans may employ either immersive or spray techniques. In an immersion chemical clean, the wafers are lowered into a pool of acid that cleans the slurry residuals from the wafers. In a spray chemical clean, an acid is sprayed across the wafers to clean the slurry residuals. Both of these types of cleaning operations are well known in the art. For instance, a wide variety of chemical spray cleaners are available from:
Exemplary chemical spray cleaners available from Semitool, Inc. include those sold under the names Magnums(copyright) and Spray Acid Tool.
More particularly, in spray cleaning, a cassette (or xe2x80x9cboatxe2x80x9d) holds the wafers and is securely placed on a rotor housed in a roughly circular chamber. Once the door to the chamber is closed, the rotor is spun at rates as high as 1,000 rpm. The spinning wafers are rinsed with a hot de-ionized (xe2x80x9cDIxe2x80x9d) water and dried with a high purity nitrogen (N2). A heated acid is then sprayed across the wafers as the rotor spins. The particular acid used depends on the chemical composition of the slurry. Depending on the particular process, additional rinsing/cleaning operations may be performed on the wafers as they are spun in the cassette on the rotor. Finally, the wafers are again rinsed with DI, the rotor is stopped, and the wafers are removed from the chamber.
Conventional rotor design for chemical spray designs suffers from several problems. One of these problems is structural and another is operational. More particularly, one problem arises from the materials from which the rotors are constructed and the radial clearance between the rotor and the spray nozzles. A second problem arises from the need for routine maintenance and upkeep.
First, the rotors are typically machined from a solid block of poly-tetra-fluoroethelene (xe2x80x9cPTFExe2x80x9d) such as is sold under the mark Teflon.(copyright) At the same time, a minimal radial clearance between the wafers and the spray nozzles is also desirable to obtain a more uniform spray pattern for fluids across the wafers. However, PTFE is not very dimensionally or thermally stable. During the cleaning operation, the temperatures involved frequently result in an inadequate radial clearance between the rotor and the spray nozzles, causing the rotor to scrape the nozzles. Even a single scrape can cause the rotor to slough PTFE particles that can contaminate the wafers, necessitating additional clean-up or even that the wafers be discarded. The scraping also aggravates variations in rotor rotation caused by load imbalances and other factors, which causes still additional scraping. Ultimately, the rotor self-destructs from the scraping and rotational variations, and this occurs more quickly as the rotor speeds increase.
Several approaches may be employed to resolve this problem. The rinse nozzles can be exchanged for shorter versions to increase the radial clearance between the rotor and the nozzles. Unfortunately, this increases the radial distance between the wafers and the nozzles and lessens the uniformity in the spray pattern on the wafers. The PTFE from which the rotor is machined can be cured prior to final machining to impart greater rigidity and more resistance to thermal expansion. However, this approach drives up the costs of the rotors. Additional steps may be taken to improve the balance of the rotors, but these steps increase the likelihood of additional problems and drives up the cost of the machine. Thus, while these approaches can reduce rotor scraping and consequent PTFE particle contamination, they adversely impact the cost and performance of the operation in other ways.
Second, the single-piece body for the rotors inhibits cleaning and repair. Because the rotors are typically constructed from a single block of PTFE, they cannot be repaired or cleaned simply by replacing clogged, scraped, or worn portions thereof They must instead be completely replaced. Even standard PTFE rotors used in conventional operations can cost several thousand dollars to replace. If cured to help preserve radial clearance as discussed above, these costs can go even higher. Attempts to construct rotors from multiple pieces resulted in rotors that lost their structural integrity and came apart at the high rotor spin speeds used in conventional chemical spray cleaning operations. Some of these multi-piece rotors were constructed from steel, but design constraints imposed by the size of the chamber resulted in a rotor that still could not remain operational.
The present invention is directed to resolving one or all of the problems mentioned above.
The invention, in a first aspect is a multi-piece rotor for use in a chemical spray cleaner. The rotor comprises a top plate, a bottom plate, a plurality of struts, and a pair of retainer bars. The top plate defining an aperture keyed to a wafer cassette. The bottom plate includes an interior surface on which the wafer cassette may bottom and an exterior surface defining an aperture for receiving a rotating shaft. The struts separate the top and bottom plates, each strut fastened at a top end thereof to the top plate and at a bottom end to the bottom plate. The retaining bars fasten to adjacent ones of the struts on the radially proximal surface thereof. In a second aspect, the invention is a rotor constructed from polyphenelyne sulfide for use in a chemical spray cleaner.